Pumps of the type discussed here are known. A pump of this kind comprises a cam ring, which has an axial aperture, in which a rotor is received so as to be rotatable relative to the cam ring. Said rotor has radial recesses, in which delivery elements, in particular vanes or rollers, are displaceably received as viewed in a radial direction. Along its circumference, the axial aperture has an inner wall having a contour on which the delivery elements run during rotation of the rotor. In this case, the contour is designed in such a way that each delivery element is moved to a greater or lesser extent out of the radial recess in which it is received, depending on its rotational position relative to the cam ring. Delivery cells delimited by adjacent delivery elements and the inner wall are thereby formed, said cells being referred to as vane cells or roller cells, depending on the type of delivery element. The volume of a vane cell of this kind varies periodically with the rotation of the rotor relative to the cam ring. In this way, at least one suction region and at least one pressure region of the pump are defined, wherein the suction region is arranged in a region in which the volume of the delivery cells increases with the revolution of the rotor. The pressure region is arranged in a region in which the volume of the delivery cells decreases with rotation of the rotor. It is possible for the pump to have two pump sections, each of which has a suction region and a pressure region. This is then what is referred to as a double-stroke pump. The pump furthermore comprises a side plate, which closes off the axial aperture on a first side, and a pressure plate, which closes off the axial aperture on a second side. In this arrangement, the side plate and the pressure plate likewise delimit the delivery cells. The pressure plate has at least one opening, through which the at least one pressure region of the pump is fluidically connected to external surroundings of the pressure plate, in particular to a “pressure chamber”. Overall, the pump thus delivers a fluid from a suction chamber fluidically connected to the suction region, via the pressure region, into the pressure chamber. The suction chamber is preferably fluidically connected to a storage tank. The pressure chamber is preferably fluidically connected to a consuming unit.
When the pump is stationary, some of the delivery elements enter the associated recesses owing to the gravitational force acting upon them, depending on the arrangement of the elements. They are then no longer resting on a cam ring wall defining the inner contour. When the pump restarts, the corresponding delivery elements are therefore not able to contribute to the delivery of the fluid until they have moved out of their recesses owing to the centrifugal force and/or through fluid pressure assistance. In order to increase the contact force of the delivery elements on the inner wall of the cam ring, a fluid path is provided from the at least one pressure region of the pump to an “under-vane” region of the delivery elements. This region is arranged radially to the inside of the delivery elements and comprises regions of the radial recesses in the rotor which are arranged radially behind the delivery elements, i.e. closer to an axis of rotation of the pump than said elements. Pressurized fluid delivered by the pump is thus passed out of the pressure region into the under-vane region and supports the delivery elements by increasing the contact force acting thereon against the inner wall of the cam ring.
In order to improve the characteristics of the pump during startup, i.e. what are referred to as cold starting characteristics, a cold-start plate is provided, which is preloaded against the pressure plate by means of a spring element in such a way that, at least when the pump is stationary, said cold-start plate closes the at least one opening in the pressure plate to the external surroundings of said pressure plate. There is then no fluid connection from the pressure region to the pressure chamber. All the fluid delivered by the pump during the starting thereof therefore passes from the pressure region, via the fluid path, into the under-vane region, with the result that all the fluid delivered is initially used to move the delivery elements out of the recesses thereof and to press them against the inner wall of the cam ring. Once the pump has thereby been brought to a fully functional state, the delivery pressure in the pressure region rises, thereby pushing the cold-start plate away from the pressure plate against the preloading force of the spring element. The fluid connection between the pressure region and the pressure chamber is thereby opened and the pump delivers fluid from the suction chamber to the pressure chamber.
EP 0 758 716 B1 discloses a pump having a corresponding supply to an under-vane region and a cold-start plate. In this case, the spring element preloading the cold-start plate is supported on a casing of the pump in order to introduce preloading forces into the cold-start plate. In the case of pumps which do not have a casing or in which the spring element cannot be supported on the casing for other reasons, complex designs are required in order to provide support for the spring element. Moreover, the individual elements, such as the cold-start plate, spring element and, if appropriate, supporting elements for the spring element are easily lost during dispatch, transportation or installation of the pump or are not secured reliably on the pump if they are not integrated into the casing in a conventional manner. This is particularly problematic in the case of pumps referred to as cartridge pumps, which themselves have no casing but are designed as pump inserts that are inserted into preprepared installation spaces in gear casings, for example.
It is therefore the underlying object of the invention to provide a pump which does not have the problems mentioned.